1. Field of the Invention
The present invention relates to a technique for coding and decoding speech signals in a communication system such as, for example, a mobile communication system or the like.
2. Description of the Related Art
In a conversation on a telephone, time for speaking from a speaker and time for hearing speech from the other party, alternately exists. Here, when the speaker hears the voice of the other party of the communication, information sent to the other party becomes "unvoice. In other words, since they are not speaking not much information is required. Thus, there is proposed a variable rate type speech coding and decoding system which heightens a transmission rate at the time of voice (at the time of speaking), and which lowers the transmission rate at the time of listening (at the time of hearing the voice of the other party) (refer to the reference described below). According to this coding and decoding method, there is provided an advantage of lowering an average transmission rate while maintaining a speech quality.
Reference: DeJaco A., Gardner W., Jacobs P., Lee C. "QCELP": The North American CDMA Digital Cellular Variable Rate Speech Coding Standard", IEEE Workshop on Speech Coding for Telecommun., pp. 5-6 (1993).
The variable rate type coding and decoding method can provide a large advantage when using a transmission channel which allows a variable transmission volume. However, when a transmission channel having a fixed transmission volume is used, the amount of information transmitted by the transmission channel is small, a fixed amount of the transmission channel will be occupied with the result that there is no meaning in that the transmission channel is rendered variable.
Furthermore, in the speech coding and decoding method, a favorable speech quality can be obtained in the case where a correlation between the quantification code used in coding and the speech information is favorable while a sufficient speech quality can not be obtained in the case where the correlation is poor.